Reyes was born in Hesperia, California, on December 26, 1989, and is of Mexican American descent. Reyes was an athlete from a young age: he wrestled and played American football, aspiring to play in the NFL. Reyes grew up in a poor family; his parents encouraged him to pursue sports in order to avoid getting involved with gangs. After graduating from Hesperia High School, he moved to New York to attend Stony Brook University, where he earned his B.S. in Information Systems.

Reyes was the starting safety for the Stony Brook Seawolves from 2009 to 2012, eventually becoming captain. He twice made the All-Conference team, including being named First Team All-Big South in 2012. He intercepted a pass in the end zone with 54 seconds remaining in the first round of the 2011 FCS Playoffs to give Stony Brook a 31–28 win over in-state rivals Albany. Reyes graduated from Stony Brook as the program’s all-time leader in solo tackles (158), recording 259 tackles in total. While he received attention from NFL teams, his speed was deemed ‘average’ and he ultimately went undrafted in the 2013 NFL Draft.

In 2017, Reyes accepted a job as an IT Technical Support Specialist at Oak Hills High School in Oak Hills, California. He left the job after two years to focus on UFC full-time. ESPN’s Hallie Grossman described his life during this time as “pulling Clark Kent-ish double duty: by day, he was the technology nerd who made sure the campus internet ran smoothly and installed new computers in classrooms. By night, he was the fighter trying to carve a place for himself in MMA, in King of the Cage and Legacy Fighting Alliance and eventually, finally, the UFC.”

Reyes made his promotional debut for the UFC on June 25, 2017 against Joachim Christensen at UFC Fight Night 112. He won the bout via technical knockout in the opening minute of the fight, and he earned the Performance of the Night bonus.

Reyes faced Jeremy Kimball on December 2, 2017 at UFC 218. Reyes won the fight via submission in the first round.

Reyes faced Jared Cannonier on May 19, 2018 at UFC Fight Night 129. He won the fight via TKO in the first round.

Reyes faced Ovince Saint Preux on October 6, 2018 at UFC 229. He won the fight via unanimous decision.

Reyes faced Volkan Oezdemir on March 16, 2019 at UFC on ESPN+ 5. Reyes won the back-and-forth fight by split decision. 11 media outlets scored the fight in favor of Oezdemir while 8 media outlets scored it for Reyes.

Reyes faced Chris Weidman on October 18, 2019 at UFC on ESPN 6 in the main event. He won the fight via knockout in round one. This win earned him the Performance of the Night award.

Reyes faced Jon Jones on February 8, 2020 for the UFC Light Heavyweight Championship at UFC 247. He lost the fight via a controversial unanimous decision. 14 of 21 media outlets scored the contest for Reyes, while only 7 scored it for Jones.

Reyes faced Jan Błachowicz for the vacant UFC Light Heavyweight Championship on September 27, 2020 at UFC 253. He lost the fight via technical knockout in the second round.